Jeb Bush continues his slow decline as the most likely Republican nominee nomine. He is still over twice as likely as any other person, but he down close to 35%, from 45% just before the first Republican debate.
Sources: Betfair, Hypermind, PredictIt, http://twopointoh.predictwise.com/politics/2016RepNomination
The second level is commanded by Donald Trump and Marco Rubio. Trump is on a positive incline after leading the polls for over two months. But, he is still only 15% likely to win the nomination. Rubio, who went down for bit prior to the first debate, has settled in as the more established alternative to Bush.
Meanwhile, Ben Carson and John Kasich have surged in the polls and they are the clear third tier of possible candidates, with Carly Fiorina just behind. All three candidates got positive reviews from the first debate and this has been reflected in their poll numbers.
Technically Scott Walker is still in that third tier, but is trajectory is down after the first debate.